CHEM 1300 Lecture Notes - Lecture 9: Formal Charge, Weighted Arithmetic Mean, Nonmetal
26 Jun 2018
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Lecture 9 – Lewis Dot continued
Drawing and building methods
1. Find valence e- for each element in compound
a. Example: O = 6, C = 4 etc.
b. Use periodic table
2. Prepare the skeletal structure
a. What is bonded to what?
b. Symmetry is usually correct
c. Mindful of bond pairs (single, double)
d. Lone electron pairs must be pictured
d.i. Note: if you guess wrong chances are during the steps you will be corrected �
3. Complete octets of outer atoms by adding dots where appropriate
a. Be mindful of how many electrons are used up
4. Find remaining electrons to use up and add them to inner/core electrons
a. Give to most EN atoms first (find on periodic table if unsure about EN)
5. Confirm your octet!
a. Count electron and bonds (staying within rules unless there is an exception)
b. Make double bonds if needed
c. Common exceptions:
c.i. Expanded/incomplete and radicals
6. Calculate the charge
a. Ions get square parentheses (metal and non-metal)
b. Non-polar Covalent are not charged compounds
7. Resonance structures available to determine best fit? If so, how many? List them all. State which
are the most plausible or stable
a. Multiple correct structures for a compound, most correct ones are the resonance
structures with the least Formal Charge
Exceptions to the octet rule
●Most common in inorganic compounds
oNon-hydrocarbons
oR-chain compounds
oAcids
1. Expanded Octets (>8 e- in a single orbital)
oP, S, Noble Gases, Larger Halogens
oPrinciple QN must be larger than n=3
oFormal charge will be at or near zero
▪Will have a stable charge
oEg) XeOF4, BrCl5, SF6, SF4
oPF6, anion
2. Incomplete Octets (<8e- in a single orbital)
oRelatively common in B, Be, Al
oFormal charge near or at zero
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Document Summary
State which are the most plausible or stable: multiple correct structures for a compound, most correct ones are the resonance structures with the least formal charge. Determines stability or plausibility: hypothetical charge, not mathematically proven or pictured. Fc must equal zero when adding all components together. Fc is equal to molecular charge: must equal zero because unless they do it is unstable and it probably doesn"t exist in that structure or state. Negative charges indicate highly en atoms (halogens and chalcogens) Positive charges indicate low en atoms (internal, cations) Fc should alternate between atoms in a compound. Formula: (#valence e-) (#dots + #bonds) When multiple structures exist for a chemical compound. Equivalent: movement of electron between same element. Near-equivalent: movement of electron between different elements of compound, may change fc in process, best compound in most stable, least variation in fc. Resonance structures combine together to give a weighted average known as a hybrid. 1 bonds, not single or double.
